Condenser assembly



Aug. 16, 1932. p, SCQFIELD 1,872,511

CONDENSER AS SEMBLY Filed Aug. 8, 1930 INVENTORT PHIL/P F. SCOF/ELD. SQMMr M ATTORNEY Patented Aug. 16, 1932 UNITED STATES PATENT forrlcril PHILIP F. soorrurm, or PALO ALTO, cnmronn'in, l'nssrononro num'rz & normals,

LTD, on SAN FRANCISCO, onmronnrnfn oonronn'rronor NEVADA coNnnNsEnfiiissn-irium Application filed August s, 1330. sum-no. 478,890.

My invention relates to radio transmitters and electrical oscillation generators, and particularly to such transmitters or generators designed for operation at both high and low frequency to give efficient operation; to pro vide a condenser assembly wherein a solid dielectric condenser may be used forlow frequency capacity without having the dielectric overheated or destroyed when high frequencies are present in the circuit; and to provide a condenser assembly which is operative at substantially uniform efliciency on both high and low frequencies. Without manual adjustment.

My invention possesses numerous other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of my invention. It is to be understood that I do not limit myself to this disclosure of species of my invention, as I may adopt variant embodiments thereof Within the scope of the claims.

. The single figure of the drawing is a wiring diagram illustrating a form of my invention as applied to a combination long and short wave radio transmitter.

In radio installations for shipboard use it is frequently desirable from considerations both of space and of cost, to utilize a combination transmitter which will operate on both high and low frequency or, in otherterms, upon both short and long wave lengths. Thus, there are marine communication channels between 14: and' il meters in the short lengths below this value difl'ers markedly from that above.

Combination equipment of this character is frequently troublesome in operation.

Equipment forshipboard use should have as few controls as is possible, since multiplicity :of adjustmentsmakes maladjustment probable in increasing ratios. Experience shows thatwith such combination equipment, there is a strong tendency for the apparatus to set up high frequency oscillations, which absorb power and limit radiation, when the equipment' isadjusted for long wave transmission,

or, ifthe device is eflicient on the long wave ad ustment, low high frequency efficiency is obtained. 1

{have found that a large portion of these required, these condensers must usually have mica orother' solid dielectrics. When these condensers are used on short waves however,

they heat excessively, causing breakdown of the apparatus. Onthe other hand, even if airycondensers of sufiicienticapacity to operate effectively on' low frequencies be provided, the efficiency of the apparatus on high frequency is impaired.

For example, in circuits using blocking condensers to permitthe proper biasing potential uponthe grid of the oscillating tube, excess capacity in these condensers results in not qappare'nt from the ordinary analyses made of these circuits, but it has been repeatedly found to exist in practice.

In accordance with my invention, instead of usinga'sin-gle condenser for blocking or coupling purposes, a condenser assemblyis used comprising a low capacity condenser, preferably having an .air-dielectric, and .a circuit in parallel with this condenser comprising a high capacity condenser in series with-a resistor.

'A preferred embodiment of my-invention isshown in the diagram, which represents the circuits employed in a combination long and short Wave ship. transmitter. The circuit utilized is, in'its' essentials,-that described in the United States PatentNo. 1,625,50i granted April '19, 1927, to F; G. Simpson. Thecirfeeble-oscillations, instability, and excessive heating of the tubes. The reason for this is connect to ground.

cuit is a symmetrical one, and comprises a pair of vacuum tubes 10, 10', connected to a common filament circuit 11 supplied by the transformer 12, which derives its power from the leads 13. In order to simplify the wiring, one side of each of the filaments is grounded, as is one side of thetransformer secondary 12.

- The plates of the vacuum tubes connect through the leads 15,15, to the wave selector switches 16, 16', which aremechani'cally in ter-connected, but electrically insulated from each other. A resistor 17, 17 is connected in series with each of these selector switches, but, in the position shown, these resistors are shorted out by the'switches themselves, the circuit tracing through the leads 18, 18

'to the coils 19, 19, varying portions of which are included by the selector switch 20, which is placed at the junction of the two branches of the circuit, and to which voltage is supi plied for the tubes by a transformer 22 feeding through a high-frequency choke-coil'23. Signals are controlled by a key 25 in the primary supply circuit 26 of the transformer The grid of each a the tubes 10 and we excited from the plate of the other tube.

A lead 27, 27 connects with the lead 18, 18, and thence to a condenser assembly which a resistor 33, 33. Grid bias for the tubes is determined by the grid leaks 35, 35, which connect together and whose common. points The circuit is tuned by a variable condenser 36. In practice, the tuning condenser engages the same sets of stator plates as are used for the condensers 28, 28, but this arrangement is for convenience only, and separate stator plates may beused if desired. It will be seen that the condenser 36 is-bridged across the two inductors 19 in series, and these inductors are arranged to have 'zero coupling. This results in a circuit wherein each grid is excited from the plate of the opposite tube, as was stated above, and no inductive coupling is required between the grid and plate circuits of the individual tubes in order to cause oscillation. The circuit is a very stable one, is particularly adapted toshort waves, but. is inoperative if either tube be removed from the circuit.

For transmission on the longer waves, the

- selector switch 16, 16 is moved to one of the i and connects the plates with the long wave coils 40, 10', which, like the short wave coils, are positioned for zero mutual coupling. The selector switch 20 is thrown to the open circuit position, disconnecting the coils 19, 19, and opening the supply circuit through the choke 23. v r

V Y The coils i0, 40 areconnected together and fed at their junction point through the low radio frequency choke-coil tl, which connects through the lead 42 tothe transform- The selector switches 16, 16 are arranged to connect with various taps on the coils 4:0,

i0, and simultaneously to connect a fixed condenser 43 across the two coils in series. Thls condenser 1s designed to glve the proper long wave ranges when connected across the tappedcoilsiO, 40 It is in parallel with the condenser 36, withwhich the fine gradations in tuning are adjusted.

"A coupling coil 45,.having'an adjustable mutual. inductance with the short wave coil, 19 transfers energy from the oscillator to the-antenna circuit. One end of this coil is grounded. The other end connects through the leads 46 with one point of a multi-point selector switch 47. Connected to the lead 46 is the rotor 48 of a condenser having one set plates 51 connects through the lead 52 to another point on the multi-point switch 47. By throwing the switch 4:? from one to the other of the two points thus .far mentioned, the

double statorcondenser may be. arranged either in series or in parallel with the coil 45. From the switch 47 the circuit traces through the lead 54:, a switch 56 whose purpose is to throw a high frequency ammeter 57 either .into or out of the antenna circuit, thence through a switch 58 and a loading coil 59 to the antenna- 60. lVhen desired, the switch 58 may be thrown to ground the antenna.

For long waves the coil 62 is coupled to the coil 40', and. various taps on the coil 62 may be selected by the multi-point switch 47.

The performance of a combined short and long wave transmitter. of this character is largely dependent upon two of the features heredescribed. Fir-stare the resistors 17,

17, which are included in the circuit when .the' selector switches are set for long Wave j which may be set up in the circuit. f rThe other feature upon which the satisfactory oscillation of. the transmitter is largely dependent is the condenser assembly comprising the air condensers 28, 28, the

condensers 32, 32, and resistors33, 33'. For

long wave operation, relativelysmall power can be transferred through the condenser 28 ,without a large voltage drop. Sufficient energy to excite the grid may, however, be transmitted through the condenser 32, 32, which may be mica condensers of from 250 to 500 micro-microfarads, without producing an excessive drop, in spite of the series resistors 33, 33 which may be of the order of from 50 to 100 ohms.

For short wave operation, the capacity of the condenser 28, 28' is ample to transmit the required power, while the use of a larger condenser, unprotected by a resistor, results in decreased efficiency and low power output. Moreover, mica condensers heat excessively and break down when subjected to the high frequencies involved, unless the voltages to which they are subjected are reduced to very small value. The resistors 33, 33 reduce the duty upon the condensers 32, 32, preventing the breakdown of the condensers and increasing the efficiency of the oscillating system to substantially the same value as were only the condensers 28, 28 used.

The most convenient type of resistor to use in the circuit is the commercial vitrified enameled resistor. Resistors of this type are advertised as substantially non-inductive, but this description does not hold at extremely high frequency. The actual effect is that of a choke-coil of very high resistance, which practically prevents current flow at these frequencies.

The action of the circuit is not dependent upon using the exact type of resistor described, however. Even with the inductance of the resistors 33, .33 reduced to a minimum, the action described takes place to a certain extent, although not so satisfactorily as under the conditions given.

It is to be understood that the usefulness of this type of condenser assembly is not limited to the circuit described nor to a grid blocking condenser assembly in any circuit. but it may be used as a coupling or blocking condenser in many positions where such are required.

The economic saving in the use of my invention is material. In the combination transmitter here described a reduction of about sixty dollars in manufacturing cost was effected by its adoption.

I claim:

1. In an electrical oscillation generator operative optionally at either low or high frequencies, an oscillating circuit, a vacuum tube having grid and plate elements connected to said circuit, and a blocking condenser assembly comprising a 10W capacity condenser and a circuit in parallel therewith comprising a condenser and a resistor in series, for maintaining said grid and plate elements at different mean operating potentials.

2. In an electrical oscillation generator operative optionally at either low or high frequencies, an oscillating circuit, a vacuum tube having grid and plate elements connected to said circuit, and a blocking condenser assembly comprising an air-dielectriccondenser and a circuit in parallel therewith comprising a solid dielectric condenser and a resistor in series, for maintaining said grid and plate elements at difierent mean operating potentials.

3; In combination with a variable frequency vacuum tube oscillator, a grid blocking condenser assembly comprising a variable air condenser, and a circuit in parallel therewith comprising a fixed condenser in series with a resistor.

4. In combination with a variable frequency oscillator, a grid coupling network comprising a high capacitybranch and a low capacity branch, said high capacity branch including a resistance, whereby the proportion of the total current through said network which traverses said low capacity branch is greater at high frequencies than at low frequencies.

'5. In combination with a variable frequency vacuumtube oscillator, a grid coupling network comprising a condenser havi my hand.

PHILIP F. SCOFIELD. 

